1. Field of the Invention
The present invention relates to organic electroluminescent devices, and more particularly, to a top emission type active matrix organic electroluminescent device.
2. Discussion of the Related Art
Flat panel display devices—which are characterized as being thin, light weight and energy efficient—are in high demand in the display field as the information age rapidly evolves. A flat panel display device can be classified into one of two types depending on whether it emits or receives light. One type is a light-emitting type display device that emits light to display images, and the other type is a light-receiving type display device that uses an external light source to display images. Plasma display panels, field emission display devices, and electroluminescence display devices are examples of the light-emitting type display devices. Liquid crystal displays are examples of the light-receiving type display device.
Among the flat panel display devices, liquid crystal display (LCD) devices are widely used for laptop computers and desktop monitors because of their high resolution, good color rendering and superior image quality. However, the LCD device has some disadvantages, such as poor contrast ratio, narrow viewing angle, and difficulty in enlarging it to a very large size having millions of pixels. Therefore, new types of flat panel displays are needed to overcome the aforementioned disadvantages, but yet are still thin, light weight and have low power consumption.
Recently, organic electroluminescent display (OED) devices have been of the most interest in research and development because they are light-emitting type display devices having a wide viewing angle and a good contrast ratio as compared to the LCD device. The organic electroluminescent display device is a light-emitting type display device that does not require a backlight device, and can be light weight and thin. Further, the organic electroluminescent display device has low power consumption. A low voltage direct current can be used to drive the organic electroluminescent display device while obtaining a rapid response speed. As widely known, since the organic electroluminescent display device is totally in solid phase, unlike the LCD device, an OED device is sufficiently strong to withstand external impacts and has a greater operational temperature range. In addition, the organic electroluminescent display device can be manufactured at a lower cost than a LCD device. Moreover, since only the deposition and encapsulation apparatuses are necessary without having to inject liquid crystal in a process of manufacturing the organic electroluminescent display device, process management is simpler than in the manufacture of LCD devices.
One operating method for the organic electroluminescent display device is a passive matrix operating method that does not utilize thin film transistors. In this type of organic electroluminescent display device, scanning lines and signal lines, which are arranged in a matrix pattern, perpendicularly cross each other. A scanning voltage is sequentially applied to the scanning lines to operate each pixel. To obtain a required average luminance, the instantaneous luminances of each pixel during a selected period is intensified by increasing the number of scans during the period.
Another method of operating an organic electroluminescent display device is an active matrix operating method. The active matrix type organic electroluminescent display device usually includes thin film transistor pairs, which create a voltage storing capability for each of the pixels. The pair of thin film transistors includes a selection transistor and a drive transistor. The source/drain of the selection transistor is connected to a signal line for supplying a data signal when a scanning signal is applied to the gate scanning line. The gate of the drive transistor is connected to the drain/source of the selection transistor. A constant voltage line is connected to the source/drain of the drive transistor. In the structure of an active matrix type organic electroluminescent display device, a voltage applied to the pixels is stored in storage capacitors, thereby maintaining the signals until the next period for applying a signal voltage. As a result, a substantially constant current flows through the pixels, and the organic electroluminescent display device emits light at a substantially constant luminance during one frame period. Because a very low current is applied to each pixel of an active matrix type organic electroluminescent display, it is possible to enlarge the display device, thereby forming much finer and/or larger patterns having low power consumption.
The driving principle for a display apparatus according to the related art will now be described in FIG. 1 that shows an equivalent circuit diagram of pixels in the active matrix type organic electroluminescent display device in the related art. As shown in FIG. 1, scanning lines are arranged in a transverse direction, and signal lines are arranged in a longitudinal direction perpendicular to the scanning lines. A power supply line that is connected to a power supply provides a voltage to drive transistors and is also disposed in the longitudinal direction. A pixel is defined between a pair of signal lines and a pair of scanning lines. Each selection transistor, otherwise known as a switching thin film transistor (TFT), is disposed in the pixel near the crossing of the scanning line and signal line and acts as an addressing element that controls the voltage of a pixel. A storage capacitor CST is connected to the power supply line and the drain/source of the switching TFT. Each drive transistor, otherwise known as a driving TFT has a gate electrode connected to the storage capacitor CST and a source/drain connected to the power supply line and acts as a current source element for the pixel. An organic electroluminescent diode is connected to the drain/source of drive transistor.
The organic electroluminescent diode has a multi-layer structure of organic thin films between an anode electrode and a cathode electrode. When forward current is applied to the organic electroluminescent diode, electron-hole pairs combine in an organic electroluminescent layer as a result of a P-N junction between the anode electrode, which provides holes, and the cathode electrode, which provides electrons. The electron-hole pairs have a lower energy together when combined than when they were separated. The energy gap between combined and separated electron-hole pairs is converted into light by an organic electroluminescent element. That is, the organic electroluminescent layer emits the energy generated due to the recombination of electrons and holes when a current flows.
Organic electroluminescent devices are classified into a top emission type and a bottom emission type in accordance with a progressive direction of light emitted from the organic electroluminescent diode. In the bottom emission type device, light is emitted in a direction toward the substrate where the various lines and TFTs are disposed. However, in the top emission type device, light is emitted in a direction opposite to the substrate where the lines and TFTs are disposed.
FIG. 2 is a partial cross-sectional view of a bottom emission type organic electroluminescent device showing one pixel having red (R), green (G) and blue (B) sub-pixels regions according to the related art. As shown in FIG. 2, first and second substrates 10 and 30 are spaced apart from each other. The first and second substrates 10 and 30 are attached to each other and sealed by a seal pattern 40. Thin film transistors T and first electrodes 12 are formed on the first substrate 10, which is transparent. The pixel of the organic electroluminescent device generally includes three sub-pixel regions with the thin film transistor T and the first electrode 12 disposed in each sub-pixel region. An organic electroluminescent layer 14 is formed over the thin film transistors T and over the first electrodes 12. The organic electroluminescent layer 14 includes luminous materials that produce red (R), green (G) and blue (B) colors each corresponding to each thin film transistor T in each sub-pixel region. A second electrode 16 is formed on the organic electroluminescent layer 14. The first and second electrodes 12 and 16 supply the electric charges to the organic electroluminescent layer 14.
The seal pattern 40 attaches the first and second substrates 10 and 30 and maintains a cell gap between the first and second substrates 10 and 30. Furthermore, although not shown in FIG. 2, a hydroscopic material or a moisture absorbent material can be formed on an inner surface of the second substrate 30 in order to absorb the moisture within the cell gap between the first and second substrates 10 and 30 to protect the cell gap from moisture. In addition, a translucent tape may be interposed between the second substrate 30 and the hydroscopic material to tightly adhere the hydroscopic material to the second substrate 30.
In the related art shown in FIG. 2, if the first electrode 12 is anode and the second electrode 16 is cathode, the first electrode 12 is formed of a transparent conductive material and the second electrode 16 is formed of a metal having a small work function. The organic electroluminescent layer 14 includes a hole injection layer 14a, a hole transporting layer 14b, an emission layer 14c and an electron transporting layer 14d in sequential order from the first electrode 12. As mentioned before, the emission layer 14c includes luminous materials that emit red (R), green (G) and blue (B) colors in the corresponding sub-pixel regions.
FIG. 3 is an enlarged cross-sectional view of one pixel region of the bottom emission type organic electroluminescent display device shown in FIG. 2. In FIG. 3, an organic electroluminescent display device generally includes a thin film transistor (TFT) T and an organic electroluminescent diode E in a luminous emitting area L. A buffer layer 30 is formed on a transparent substrate 1. The TFT T includes a semiconductor layer 62 on the buffer layer 30, a gate electrode 68, a source electrode 82 and a drain electrode 80. A power electrode 72 extending from the power supply line is connected to the source electrode 80, and the organic electroluminescent diode E is connected to the drain electrode 82. A capacitor electrode 64 made of the same material as the semiconductor layer 62 is disposed below the power electrode 72. The power electrode 72 corresponds to the capacitor electrode 64, and an insulator is interposed therebetween, thereby forming a storage capacitor CST.
The organic electroluminescent diode E includes the first electrode 12, the second electrode 16 and the organic electroluminescent layer 14 interposed between the first electrode 12 and the second electrode 16. The organic electroluminescent device shown in FIG. 3 has a luminous area L where the organic electroluminescent diode E emits light produced therein. Furthermore, the organic electroluminescent display device has array elements A that include the TFT T, the storage capacitor CST, the various lines and the various insulators, and on which the organic electroluminescent diode E is disposed. In the related art shown in FIG. 3, the organic electroluminescent diode E and the array elements A are formed on the same substrate.
FIG. 4 is a flow chart illustrating a fabrication process of an organic electroluminescent device of FIG. 3 according to the related art. Step st1 denotes a process of forming the array elements on the first substrate in which the first substrate is a transparent substrate. For example, the scanning lines, the signal lines and the switching and driving thin film transistors are formed on and over the first substrate. The signal lines are formed perpendicularly across the scanning lines. Each of the switching thin film transistors is disposed near a crossing of the scanning and signal lines. The formation of the array elements also includes forming the storage capacitors and the power supply lines.
In step st2 of FIG. 4, the first electrode of the organic electroluminescent diode is formed. The first electrode is in each sub-pixel region. The first electrode is also connected to the drain/source of the driving thin film transistor in each sub-pixel region.
In step st3 of FIG. 4, the organic electroluminescent layer is formed on the first electrode. If the first electrode is the anode, the organic electroluminescent layer is formed to have a sequential multiple structure of a hole injection layer, a hole transporting layer, an emission layer and an electron transporting layer on the first electrode. If the first electrode is the cathode, the sequence is reversed.
In step st4 of FIG. 4, the second electrode of the organic electroluminescent diode is formed on the organic electroluminescent layer. The second electrode covers the entire surface of the first substrate. The second electrode also acts as a common electrode.
Step st5 of FIG. 5 is a process step for encapsulating the first and second substrates. In this step st5, a second substrate is attached to the first substrate having the array elements and the organic electroluminescent diode. The second substrate protects the organic electroluminescent diode of the first substrate from external impacts; Because the first substrate is encapsulated with the second substrate, the organic electroluminescent diode is protected from the outer atmosphere. As mentioned before, the second substrate can have the hydroscopic material on the inner surface thereof.
The yield of array elements depends on the yield of organic electroluminescent layer. The fabrication yield of organic electroluminescent layer determines and controls the total fabrication yield of organic electroluminescent layer. For example, although the thin film transistors are formed without any defects on the first substrate, the first substrate having both the array elements and the organic electroluminescent layer is decided to be an inferior product if some defects occur in later processes for forming the organic electroluminescent layer. Thus, it is a waste of time and cost to fabricate the array substrate on the first substrate when defects later occur in the organic electroluminescent layer during the fabrication.
Moreover, in the bottom emission type device, light is emitted in a direction toward the substrate where the lines and TFTs are disposed. Therefore, the display area decreases because the emitted light is blocked by these lines and TFTs. In the top emission type device, since light is emitted in a direction opposite to the substrate where the lines and TFTs are disposed, the display area can increase as much as it can, and it is easy to design the TFT into a designated shape. However, since the top emission type organic electroluminescent display device of the conventional related art has the cathode electrode on the organic electroluminescent layer, the cathode electrode is generally formed of a transparent or translucent material that may block some of the light emitted from the organic electroluminescent layer that decreases light efficiency.
To prevent the decrease of the light permeability, a thin film passivation layer may be formed over the entire surface of the substrate. However, in this case of forming the thin film passivation layer, the outer atmosphere is not prevented sufficiently and may affect the organic electroluminescent diode.